This invention relates to an electrical power supply system including a source of AC power which is operable to provide an AC output voltage which has at least one phase, a series train AC/AC conversion circuit comprising first convertor means operable to establish an intermediate DC link by converting the AC output voltage into an intermediate DC voltage and second convertor means operable to convert the intermediate DC voltage into an AC output voltage having at least one phase for supply to an external load. Such an electrical power supply system is referred to in this description as an electrical power supply system of the kind referred to hereinbefore. More particularly, although not exclusively, this invention relates to an electrical power supply system of the kind referred to hereinbefore which may be an electrical power generating system and/or a motor starting system. This invention also relates to a permanent magnet generator operable to generate a variable voltage generator output, the generator being provided with rectifying means operable to rectify the variable voltage generator output.
WO 01/56133 discloses an electrical power supply system of the kind referred to hereinbefore which is an electrical power generating system and which also includes sensing means operable to monitor the intermediate DC link when the external load is connected across the AC power output and to provide a feedback signal to the speed control means whereby to effect a variation of the variable voltage generator output and thereby to counter a tendency of the intermediate DC voltage to vary. In order to provide the electrical power generating system disclosed by WO 01/56133 with protection from the consequences of a short circuit when the external load is connected across the AC power output, a trip switch would be provided in the, or each phase of the, AC power output, the or each trip switch being operable to disconnect the AC power output and the external load in response to short circuit conditions being sensed. The, or each, trip switch would be arranged to trip when current flow through it has been maintained at a certain level for a certain time.
Suitable trip switches have a current/time tripping characteristic which allows a high current flow for a short time, sufficient for starting a motor or switching on lamps, before they are tripped by the high current or they allow lower levels of current flow for longer periods of time before they are tripped. The short circuited load current is liable to build up to a high level if the trip switches are arranged to be tripped by a low operating current.
Naturally high frequency self commuted devices such as transistors and other electronic components of the system need to be rated to withstand the power to which the trip switches may be subjected without tripping. Accordingly, where the second convertor means comprise a transistor arrangement, as is usual, it is desirable for the transistors thereof to be rated so as to withstand a current several times greater than the current to which they are subjected under normal load operating conditions as such a higher than normal current is needed to trip the trip switches with minimal time delay in response to the sensing of short circuit conditions. This requirement imposes a significant cost penalty as the cost of a transistor increases significantly with its current rating. Similar trip switches may be employed in a motor starting system to provide the permanent magnet motor with overcurrent protection and the foregoing observations having regard to provision of trip switches in an electrical power generating system such as is disclosed by WO 01/56133 in order to provide protection from the consequences of a short circuit when the external load is connected across the AC power output apply to the use of such trip switches for overcurrent protection in a motor starting system.
U.S. Pat. No. 6,295,215 issued Sep. 25, 2001 to Faria et al. discloses an electrical power supply system of the kind referred to hereinbefore which is provided with a bypass circuit which connects the or each phase of the AC output voltage provided by the source of AC power directly to a respective phase of the AC output voltage of the second convertor means thereby bypassing the series train AC/AC conversion circuit, the bypass circuit including an electronically operable unidirectional switch arrangement for the or each phase of the AC output voltage provided by the source of AC power, the electronically operable unidirectional switch arrangement being operable to enable current flow through the bypass circuit as well as through the series train AC/AC conversion circuit when the power supply system is connected to an external load, the electrical power supply system also including current sensing means operable to sense current flow to the external load when the power supply system is connected to that load and control means responsive to the current sensing means and operable to control the unidirectional switch arrangement and thereby to control current flow through the bypass circuit. The electrical power supply system of the kind referred to hereinbefore disclosed in U.S. Pat. No. 6,295,215 is commonly used in equipment such as uninterrupted (or “uninteruptible”) power supplies (UPSs), motor drives and other applications. In a first mode of operation of the electrical power supply system of the kind referred to hereinbefore disclosed by U.S. Pat. No. 6,295,215, the series train AC/AC conversion circuit may be operated such that it causes the bypass circuit to predominately transfer real power from the source of AC power to the external load, that real power being a second component of power from the source of AC power to the external load, the first component of power from the source of AC power to the external load being transferred by the series train AC/AC conversion circuit. In a second mode of operation, the series train AC/AC conversion circuit transfers power from the source of AC power to the external load while the electronically operable unidirectional switch arrangement of the bypass circuit is open so that no power is transferred from the source of AC power to the external load through the bypass circuit.
U.S. Patent Application, Publication No. 2003/0043521, discloses provision of a fuse between a power supply and a number of loads in parallel. The fuse is caused to be blown by a control circuit in response to sensed fault conditions. This is achieved by connecting the fuse to earth through a bypass circuit which bypasses the loads, the bypass circuit having a lower resistance so that the fuse blows.
An object of this invention is to enable the trip switch or switches provided either to protect an electrical power supply system of the kind referred to hereinbefore which is incorporated in an electrical power generating system from the consequences of a short circuit when the external load is connected across the AC power output or to provide a motor starting system with overcurrent protection, to be tripped with minimal delay after short circuit or overcurrent conditions are sensed whilst allowing low cost, low current rated electronic components to be used in the first and second convertors and in any sensing means of the electrical power supply system of the kind referred to hereinbefore.
According to one aspect of this invention there is provided an electrical power supply system including a source of AC power which is operable to provide an AC output voltage which has at least one phase, a series train AC/AC conversion circuit comprising first convertor means operable to establish an intermediate DC link by converting the AC output voltage into an intermediate DC voltage and second convertor means operable to convert the intermediate DC voltage into an AC output voltage having at least one phase for supply to an external load, a bypass circuit which connects the or each phase of the AC output voltage provided by said source directly to a respective phase of the AC output voltage of the second convertor means thereby bypassing the series train AC/AC conversion circuit, the bypass circuit including an electronically operable unidirectional switch arrangement for the or each phase of the AC output voltage provided by said source, the electronically operable unidirectional switch arrangement being operable to enable current flow through the bypass circuit as well as through the series train AC/AC conversion circuit when the power supply system is connected to an external load, current sensing means operable to sense output current flow from the series train AC/AC conversion circuit when the power supply system is connected to said load and control means responsive to said current sensing means and operable to control said unidirectional switch arrangement and thereby to control current flow through said bypass circuit, wherein a respective trip switch is provided for the or each phase of the AC output voltage of the second convertor means and the respective phase of the bypass circuit, the unidirectional switch arrangement being open normally so that normally there is no current flow through the bypass circuit but the control means being operable to close the unidirectional switch arrangement in response to the current sensing means sensing current flow through the series train AC/AC conversion circuit in excess of a predetermined current for a predetermined time so that the flow of current from the series train AC/AC conversion circuit to the external load through the trip switch or switches is augmented by the current flow through the bypass circuit whereby the or each trip switch is caused to trip by the augmented current flow through it.
According to another aspect of this invention there is provided a permanent magnet generator including two sets of star-connected multi-phase stator windings, each of the stator windings of each of the two sets being connected between diodes of a respective branch of a respective multi-phase diode rectifier arrangement which has a positive and a negative output, the positive outputs of each of the rectifier arrangements being for connection to the positive input of the first convertor means of an electrical power generating system according to said one aspect of this invention, the negative outputs of each of the rectifier arrangements being for connection to the negative input of said first convertor means and the common terminal of the two sets of star-connected multi-phase windings being for connection to the neutral of electrical power generating system.
Provision in an electrical power supply system of the kind referred to hereinbefore of a bypass circuit in accordance with said one aspect of this invention renders a permanent magnet generator arrangement according to the other aspect of this invention particularly useful as the source of AC power of the electrical supply system of the kind referred to hereinbefore.
In the preferred embodiment of this invention the electronically operable unidirectional switch arrangement is formed of thyristors which have a much higher ability to carry high currents than do transistors. Triggering of the thyristors results in the load being supplied directly from the generator by a high current which causes tripping of the trip switches and thereby protects the components of the AC/AC conversion circuit. The generator provides a variable high frequency output voltage whereas the frequency of the output voltage of the AC/AC conversion circuit is relatively low. This results in the thyristors being commutated naturally by the generator output voltage. Short pulses of current are passed by the bypass circuit, bypassing the AC/AC conversion circuit when the thyristors are tripped and several such short current pulses are passed during one half cycle of the output voltage of the AC/AC conversion circuit. The bypass circuit may be arranged to produce a large number of current pulses for each half cycle of the output from the AC/AC conversion circuit and that leads to a high quality output voltage and current from the system. This may be achieved by providing a large number of bypasses, each with its own unidirectional switch arrangement. For example, each phase of the generator output voltage may be connected to each phase of the output of the AC/AC conversion circuit via its own bypass including its own unidirectional switch arrangement. Such an arrangement is not only suitable for providing short circuit protection but it is also suitable for providing current overload protection in a motor starting system.
An electric power supply system according to one embodiment of the present invention comprises a source of AC power which is operable to provide an AC output voltage, a series train AC/AC conversion circuit including a first converter operable to establish an intermediate DC link, a bypass circuit connecting the AC output voltage provided by the source to a second converter for bypassing the series train AC/AC conversion circuit, a current sensor operable to sense output current flow from the series train AC/AC conversion circuit and a control unit responsive to the current sensor and operable to control a unidirectional switch arrangement and thereby to control current flow through the bypass circuit. Further, a trip switch is provided for an AC output voltage of the second converter and the bypass circuit wherein the unidirectional switch arrangement is normally open so that normally there is no current flow through the bypass circuit, the control unit being operable to close the unidirectional switch arrangement.
Several forms of electrical power supply systems in which this invention is embodied will be described now by way of example with reference to the accompanying drawings.